In this blog post, we will examine the “rape adaptation hypothesis” and the “byproduct hypothesis” based on the concepts of adaptation and byproducts in evolutionary biology, and summarize the various discussions surrounding them.
How can we distinguish between adaptation and byproducts?
Natural selection, as proposed by Darwin in ‘On the Origin of Species’, is central to the mechanism of evolution. Natural selection refers to the process by which, among different individuals within a population, those with traits advantageous for survival gain greater opportunities to survive and reproduce, and those traits are passed on to the next generation. In evolutionary biology, traits formed as a result of natural selection are generally referred to as “adaptations.” In other words, an adaptation is an organ or trait formed through the evolutionary process that helps an organism survive and reproduce in its habitat.
In contrast, there is the concept of a “byproduct.” A byproduct refers to a trait that did not undergo natural selection to perform a specific function, like an adaptation, but rather arose incidentally during the process of forming an adaptive trait. For example, consider the human navel. The navel does not serve a function that directly aids survival or reproduction. Therefore, it is more appropriate to view the navel as a byproduct of the umbilical cord rather than an adaptive trait. The umbilical cord, which serves as the pathway through which a fetus receives nutrients from the mother, is an adaptive trait that contributes to the survival and reproduction of the species, while the navel is a byproduct left over from that process.
So, which traits are adaptations and which are byproducts? Distinguishing between adaptations and byproducts is by no means simple, and there has long been extensive debate surrounding the criteria and methods for distinguishing between adaptive and non-adaptive traits. Theodosius Dobzhansky, an American geneticist and evolutionary biologist, defined adaptation as the evolutionary process that enables organisms to survive better in their habitats. It is believed that, as a result of adaptation, organisms have acquired adaptive traits—such as physical structure, behavior, and physiological phenomena—that have shaped their current form.
So, is rape truly an adaptation? Rape refers to a crime in which sexual acts are performed without the other person’s consent through violence or coercion. To argue that rape is an adaptation, one must prove that such behavior is the result of natural selection, has been genetically transmitted, and contributed to reproductive success in the past.
A prominent debate over whether rape should be viewed as an adaptation or a byproduct can be found in ‘A Natural History of Rape: Biological Bases of Sexual Coercion’. In this book, co-author Randy Sonhill supports the adaptation hypothesis, while another co-author, Craig Palmer, presents the byproduct hypothesis, reflecting their differing positions. Adaptationists like Sonhill, who emphasize the influence of natural selection, argue that rape is a behavior selected by natural selection to increase reproductive success, whereas anti-adaptationists, including Palmer, view it as merely a byproduct of human sexual desire.
I found the adaptationists’ explanation to be relatively more persuasive in this debate. Therefore, in this article, I intend to examine the position that rape is a heritable trait—and how it may have contributed to reproductive success—by focusing on the various pieces of evidence presented in the book.
However, before discussing this question, there is a point that must be addressed. Even if we assume that rape is an adaptation, that fact alone cannot serve as a justification for rape. In other words, the statement “rape is an adaptation” cannot lead to the conclusion that “rape is justified.” Rape is a serious crime that causes severe physical and psychological harm to the victim and must be strictly punished by law. Furthermore, examining the possibility that a certain behavior is an evolutionary adaptation is distinct from morally justifying that behavior.
What do examples from the animal kingdom show?
One of the bases for the claim that rape is an adaptation is the animal studies presented in ‘The Natural History of Rape’, mentioned earlier. The authors explain that forms of forced sexual behavior similar to human rape have been observed in a wide variety of animal species. They state that forced mating behavior has been reported in various animals, ranging from the scorpion fly (an insect) to primates such as chimpanzees and orangutans.
In the case of orangutans, instances have been observed where immature males attempt to force themselves on females. Strong, mature females may actively resist or push away such attempts. This behavior can be interpreted as forced sexual conduct rather than consensual mating.
Furthermore, in the case of scorpionflies, males induce mating by offering females solid matter from their saliva or food as a so-called “nuptial gift.” However, cases have been reported where, if these attempts fail, the males engage in forced mating behavior. Proponents of the adaptation hypothesis point to the fact that forced sexual behavior is observed in such a wide variety of animals as evidence that this behavior likely provided certain reproductive advantages and is therefore an adaptive trait inherited from a common ancestor.
Furthermore, in some insects, specialized physical structures designed for forced copulation have been observed. The male scorpionfly mentioned earlier possesses an organ similar to a clamp for holding the female; while this organ is rarely used during normal mating, it is known to play a crucial role in forced mating situations. Furthermore, research has shown that removing this organ makes forced mating difficult. Researchers supporting the adaptive hypothesis cite these findings as evidence that forced sexual behavior may be the product of structural adaptation.
The cases examined so far provide evidence suggesting that forced sexual behavior in the animal kingdom may be an adaptive trait. However, critics argue that phenomena observed in animals cannot be directly applied to humans. Nevertheless, proponents of the adaptive hypothesis point out that humans also share a common ancestor with other primates in the course of evolution and suggest that the behavior of early humans may have been quite different from that of modern human society. Therefore, it is plausible to suggest that in the past, forced sexual behavior may have been one of the strategies to increase reproductive success.
However, rape is a crime strictly prohibited by law today, and some studies suggest that even in past hunter-gatherer societies, individuals who committed rape would likely have been expelled from the community or even sentenced to death if their actions were discovered. Considering these points, rape could actually be viewed as behavior that is disadvantageous rather than advantageous for survival and reproduction. In other words, critics on the opposing side argue that, since humans are social beings, rape is actually a behavior that is disadvantageous for adaptation.
In response, proponents of the adaptation hypothesis emphasize that adaptation refers to traits formed over a very long period through natural selection. The origin of modern humans (Homo sapiens) is estimated to be about 300,000 years ago, and even before that, various human species had evolved over long periods of time. In comparison, modern nation-states and legal systems are social institutions that have emerged only very recently. Furthermore, effective contraception and safe abortion techniques are also extremely recent developments when viewed against the entire evolutionary history of humankind. Considering these points, it is plausible to argue that in the past, the likelihood of rape leading to pregnancy was much higher than it is today.
Of course, rape would have been a very risky strategy even in the past. While rape could lead to pregnancy and thus provide reproductive benefits, there was also the risk of retaliation from the victim’s family or community. Ultimately, it can be viewed as a behavior that required weighing both reproductive benefits and survival risks. The adaptation hypothesis suggests that if a male selectively assessed these risks and benefits depending on the situation, he might have had a higher chance of reproductive success than if he acted indiscriminately or did not attempt it at all.
Furthermore, it is argued that the existence of social relationships among certain group-living primates does not necessarily preclude such behavior from natural selection, as instances of forced sexual intercourse have been observed in these species. Therefore, the adaptive hypothesis maintains that it is difficult to conclude that rape was not an adaptive trait during past evolutionary processes simply because it is disadvantageous in relatively recently formed human societies.
Another counterargument points out that while some animals possess specialized physical organs that facilitate forced sexual behavior, no such organ has been discovered in humans that exists solely for the purpose of rape. Some argue that if such an organ had been identified in humans, the debate surrounding the rape adaptation hypothesis within evolutionary psychology would not have continued as it has today.
In response, some researchers have suggested the possibility that, instead of specific organs, physiological changes exist in humans. For example, Gordon Gallup, an evolutionary psychologist at the University at Albany, State University of New York, has proposed a hypothesis related to the composition of semen from rapists. He raised this possibility based on research findings showing that male semen contains follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and that semen collected in rape situations exhibited higher concentrations of luteinizing hormone.
Luteinizing hormone is known to be associated with the female ovulation process. Gordon Gallup suggested that high concentrations of luteinizing hormone might promote ovulation in women. However, it has not yet been established that sexual intercourse itself directly induces ovulation in humans, and this hypothesis cannot be considered a widely accepted conclusion within the academic community. While it has been reported that certain substances in semen promote ovulation in some animals that undergo induced ovulation—such as camels and llamas—there is not yet sufficient evidence to directly apply this finding to humans. Therefore, although such research can be presented as one argument supporting the rape adaptation hypothesis, it is appropriate to view it as a controversial hypothesis for the time being.
How convincing is the rape adaptation hypothesis?
If rape is viewed as an adaptation, how can we explain rape directed at same-sex partners, children, or postmenopausal women—individuals who are incapable of pregnancy or are not directly related to reproduction? Critics argue that since these cases appear unrelated to species survival or reproduction, it is more reasonable to understand them as byproducts of the sexual desire mechanism.
However, the adaptation hypothesis explains that adaptive behaviors are not necessarily used solely for the purpose of reproduction.
To illustrate this with another example, consider the mechanism of male ejaculation. There is generally no disagreement that ejaculation is an adaptive behavior that evolved for reproduction. However, ejaculation does not occur only in situations where a woman is impregnated. The same mechanism operates not only during sexual intercourse but also during masturbation. In other words, even a mechanism formed through adaptation is not always used solely for its original evolutionary function.
The adaptation hypothesis argues that rape behavior can be explained in a similar way. Even if rape behavior is a product of adaptation, it does not necessarily have to occur exclusively toward women of reproductive age. Therefore, the fact that rape occurs toward targets with no reproductive potential does not immediately refute the adaptation hypothesis.
We have examined various discussions regarding whether rape can be viewed as an adaptation. Proponents of the adaptation hypothesis suggest that rape may be an adaptation, citing evidence such as the observation of forced sexual behavior in various animals, the existence of specific bodily organs related to forced sexual behavior in some animals, and certain research findings related to human physiological characteristics.
On the other hand, there is no shortage of criticism. A major counterargument is that in human society, rape can actually be disadvantageous for survival and reproduction, and that various instances of rape not aimed at reproduction are difficult to explain solely through adaptation. In response, proponents of the adaptation hypothesis counter by pointing out that the history of human society is very short compared to the timescale of evolution, and that adaptive behaviors are not necessarily used solely for their original functions.
However, it is clear that the rape adaptation hypothesis remains a subject of considerable debate even within evolutionary psychology, and that it is a hypothesis that is difficult to verify directly. Therefore, rather than asserting that any one position is definitively correct, it is necessary to adopt a critical approach that examines both the evidence and limitations presented by each hypothesis.
